If the temperature of a machining fluid fluctuates during electrical discharge machining, the specific resistance of the machining fluid and hence, the machining speed, fluctuate, and also, the dimensions of a workpiece vary. As a result, the stability of the electrical discharge machining is ruined, and the machining accuracy is lowered. Conventionally, therefore, when the machining fluid temperature detected by means of a temperature sensor, such as a thermostat, exceeds an allowable upper limit value, the operation of a cooler for cooling the machining fluid is started. When the machining fluid temperature is lower than an allowable lower limit value, the operation of the cooler is stopped. Thus, the machining fluid temperature is controlled within a predetermined range of temperature.
Even in an electrical discharge machine provided with such a machining fluid temperature control system, however, if the cooler or the temperature sensor goes wrong, or if the electrical discharge machining is executed without connecting the cooler to the power supply, for example, the machining fluid temperature deviates from the predetermined temperature range, so that the aforementioned awkward situation is entailed. Namely, the stability of the electrical discharge machining is ruined if the machining fluid temperature rises too high, or the machining fluid is frozen if it is excessively cooled, for example.